Body Fluids and Osmolarity 23-24 PDF

Introduction to body fluid compartments and osmolarity Dr Kathryn Taylor Where opportunity creates success Total body water Fluid balance The basic principle of fluid balance is: fluid balance can be maintained only if intake equ...

Introduction to body fluid compartments and osmolarity Dr Kathryn Taylor Where opportunity creates success Total body water Fluid balance The basic principle of fluid balance is: fluid balance can be maintained only if intake equals output. Obviously, if more water or less leaves the body than enters it, imbalance will result. Total fluid volume will increase or decrease but cannot remain constant under these conditions Fluid and electrolyte balance Extracellular fluid makes up the internal environment of the body. It therefore serves the dual vital. functions of providing a relatively constant environment for cells and transporting substances to and from them. Intracellular fluid, on the other hand, because it is a solvent, functions to facilitate intracellular chemical reactions that maintain life. When compared according to volume, intracellular fluid is the largest (25 L), plasma the smallest (3 L), and interstitial fluid in between (12 L). Clinical relevance : It is the extracellular levels of electrolytes in the blood or blood serum that are measured clinically. Body Fluid Compartments Body fluid compartments. Intracellular fluid (ICF) is separated from extracellular fluid (ECF) by cell membranes. ECF is composed of the interstitial fluid bathing cells and the blood plasma within the vascular system. Interstitial fluid is separated from plasma by capillary endothelia. Transcellular fluid is part of the ECF and includes epithelial secretions such as the cerebrospinal and extraocular Citation: Chapter 1 General Physiology, kibble JD. The Big Picture Physiology: fluids. ECF has a high [Na+] and Medical Course & Step 1 Review, 2e; 2020. Available at: a low [K+], whereas the opposite https://accessmedicine.mhmedical.com/content.aspx? sectionid=245544050&bookid=2914 Accessed: January 11, 2021 is true of ICF. All compartments Copyright © 2021 McGraw-Hill Education. All rights reserved have the same osmolarity at steady state. What is fluid homeostasis? In the steady state, our total body water content and salt content remain constant. An increase or decrease in water and salt intake is paralleled by an equivalent change in renal water and salt excretion. Fluid homeostasis is achieved through the process of glomerular filtration of plasma to produce an ultrafiltrate. The tubules then process this ultrafiltrate so that the final urine flow rate and solute excretion meet the homeostatic needs of the body. Osmolality and osmolarity are measurements of the solute concentration of a solution. In practice, there is negligible difference between the absolute values of the different measurements. For this reason, both terms are often used interchangeably, even though they refer to different units of measurement. Alterations in water homeostasis can disturb cell size and function. Although most cells can regulate cell volume in response to osmolar stress, neurons are particularly at risk. Therefore, regulating water balance is fundamental to survival  Osmolarity is an estimation of the What is osmolar concentration of plasma and is osmolarity? proportional to the number of particles per litre of solution; it is expressed as mmol/L. This is what is used when a calculated value is derived. It is derived from the measured Na+, K+, urea and glucose concentrations. The osmolarity is unreliable in various conditions - eg, pseudohyponatraemia such as hyperlipidaemia in nephrotic syndrome, or hyperproteinaemia.  Osmolality is an estimation of the What is osmolar concentration of plasma and is osmolality? Osmolality and Osmolarity. proportional to the number of particles per kilogram of solvent; it is expressed as mOsmol/kg (the SI unit is The osmolal concentration of a solution is called osmolality when mmol/kg but mOsmol/kg is still widely the concentration is expressed as used). This is what is used when values osmoles per kilogram of water; it are measured by a laboratory. is called osmolarity when it is Osmolality is measured by clinical expressed as osmoles per litre of laboratories using an osmometer - solution. In dilute solutions such either a freezing point depression as the body fluids, these two terms can be used almost osmometer or a vapour pressure synonymously because the depression osmometer. The normal differences are small. In most osmolality of extracellular fluid is 280- cases, it is easier to express body 295 mOsmol/kg. fluid quantities in litres of fluid rather than in kilograms of water. Therefore, most of the calculations used clinically are Osmosis Osmosis is the net diffusion of water across a selectively permeable membrane from a region of high water concen- tration to one that has a lower water concentration. What is happening to these cells? Effects of isotonic, hypertonic and hypertonic solutions on cell volume Osmolarity Osmolarity is the total solute concentration within a specific volume of a solvent expressed in osmoles per liter (Osm/L) or milliosmoles per liter (mOsm/L). These solute (dissolved particle) concentrations must be osmotically active – that is, they cause the movement of water across a selectively permeable membrane (the cell membrane) via osmosis. Osmoles are measured concentrations of dissociated ions in a solution that contribute to osmotic pressure.  ‘An electrolyte as a compound that will break up or What is an dissociate into charged particles called ions when placed in solution. Sodium chloride, when dissolved in water, provides a positively charged electrolyte? sodium ion (Na + ) and a negatively charged chloride ion (Cl − )’ ‘If two electrodes charged with a weak current are placed in an electrolyte solution, the ions will move, or migrate, in opposite directions according to their charge. Positive ions such as Na + will be attracted to the negative electrode (cathode) and are called cations. Negative ions such as Cl − will migrate to the positive electrode (anode) and are called anions. Various anions and cations serve critical nutrient or regulatory roles in the body. Important cations include sodium (Na + ), calcium (Ca ++ ), potassium (K + ), and magnesium (Mg ++ ). Important anions include chloride (Cl − ), bicarbonate (HCO 3 − ), phosphate (HPO 4 = ), and many proteins’ What causes changes to the ECF and ICF volumes Some of the different factors that can cause extracellular and intracellular volumes to change markedly are ingestion of water, dehydration, intravenous infusion of different types of solutions, loss of large amounts of fluid from the gastrointestinal tract, and loss of abnormal amounts of fluid by sweating or through the kidneys. Isosmotic , hyperosmotic, hyposmotic changes can result from these changes Shifts of water between body fluid compartments Disturbances of body fluid can cause Volume contraction: A decrease in ECF i.e. volume depletion volume Volume expansion: An increase in ECF volume i.e. volume overload. The terms isosmotic, hyposmotic and hyperosmotic refer to the osmolarity of the ECF. Clinical Abnormalities of Fluid Volume Regulation: Hyponatremia and Hypernatremia A measurement that is readily available to the clinician for evaluating a patient’s fluid status is the plasma sodium concentration. Plasma osmolarity is not routinely measured but, because sodium and its associated anions (mainly chloride) account for more than 90% of the solute in the extracellular fluid, plasma sodium concentration is a reasonable indicator of plasma osmolarity under many conditions. When plasma sodium concentration is reduced more than a few milliequivalents below normal (about 142 mEq/L), a person is said to have hyponatremia. When plasma sodium concentration is elevated above normal, a person is said to have hypernatremia Causes of Hyponatremia: Excess Water or Loss of Sodium Decreased plasma sodium concentration can result from loss of sodium from the extracellular fluid or addition of excess water to the extracellular fluid A primary loss of sodium usually results in hyponatremia and dehydration and is associated with decreased extracellular fluid volume. Conditions that can cause hyponatremia as a result of the loss of sodium include diarrhea and vomiting. Overuse of diuretics that inhibit the ability of the kidneys to conserve sodium and certain types of sodium-wasting kidney diseases can also cause modest degrees of hyponatremia. Addison disease , which results from decreased secretion of the hormone aldosterone, impairs the ability of the kidneys to reabsorb sodium and can cause a modest degree of hyponatremia. Regulation of Body Fluid Compartments : Extracellular and Intracellular Fluids; Edema Hall, John E., PhD, Guyton and Hall Textbook of Medical Physiology, Chapter 25, 305-320 Brain cell volume regulation during hyponatremia. During acute hyponatremia, caused by loss of Na + or excess H 2O, there is diffusion of H 2O into the cells (1) and swelling of the brain tissue (indicated by the dashed lines). Copyright © 2021 Copyright © 2021 by Elsevier, Inc. All rights reserved. Plasma Na + concentration Water and Electrolytes Homeostasis Water and Electrolytes Homeostasis Dominiczak, Marek H., Medical Biochemistry, Chapter 35, 523-537 Potassium balance. Serum potassium concentration is maintained within narrow limits. Both low (hypokalemia) and high (hyperkalemia) concentrations are dangerous because potassium affects the contractility of heart muscle. Copyright © 2019 © 2019, Elsevier Limited. All rights reserved.